Natural gas streams containing carbon dioxide in high concentrations occur naturally and as the result of enhanced oil recovery (EOR) operations. During EOR operations, carbon dioxide is injected into mature oil fields to improve the miscibility of the oil and enhance its production to the surface. Gas returning to the surface, along with the oil, contains hydrocarbon gases and carbon dioxide. The carbon dioxide is separated from the hydrocarbon gases and reinjected. The hydrocarbon stream is further treated and either used for fuel gas or sold to a pipeline.
Heretofore, attempts to recover carbon dioxide from associated gases were by processes using a chemical solvent to scrub the gas and thereafter distilling to recover the carbon dioxide. Such methods were costly and met with problems regarding corrosion and solvent degradation. Examples of such processes and methods to alleviate the corrosion problems are found in U.S. Pat. No. 2,065,112 which issued in 1936, U.S. Pat. No. 2,399,142 which issued in 1946, U.S. Pat. No. 2,377,966 which issued in 1945, U.S. Pat. No. 4,477,419 which issued in 1984, U.S. Pat. No. 3,137,654 which issued in 1964. The alkanolamine process described in some of these patents is still widely used today.
More recently, commercial membrane technology has been developed for separating acid gases like carbon dioxide and hydrogen sulfide from light hydrocarbon gases. Examples of such art are found in U.S. Pat. No. 4,130,403 which issued Dec. 19, 1978 to T. E. Cooley et. al., U.S. Pat. No. 4,639,257 which issued Jan. 27, 1987 to Melvyn Duckett et. al., and U.S. Pat. No. 5,233,837 which issued Aug. 10, 1993 to Richard Callahan. The features of these patents, particularly the use of membrane separation, was a definite step forward in the carbon dioxide separation art. However, the prior art in this field continues to suffer from overall efficiency of the process and undesirable membrane life. Furthermore, the prior art is not readily adapted to low feed stream pressures.
Accordingly, a continuing search has been directed to the development of a system and method which can separate carbon dioxide from natural gas, particularly in low pressure EOR processes.